1. Field of the Invention
The invention relates to a proportional counter used as a measuring probe for the detection and measurement of radioactive surface contaminations comprising a case with thin-walled radiation entrance window made of a film or thin foil fixed to the case along the film edge and with a "contact safety device" protecting the window against accidental contact and with a connecting base or cap.
2. Description of the Prior Art
When working with radioactive substances, in particular with .alpha.- and .beta.- emitters it is imperative to check objects, facilities, equipment, workplaces etc. for radioactive contamination of their surfaces, i.e. the surface contamination must be determined. Such checking is legally required both in radiation protection monitoring and for the release of materials from nuclear facilities for harmless further utilization. Although large quantities of material have accumulated worldwide as socalled radioactive wastes, there is a lack of suitable measuring equipment, in particular of suitable detectors for measuring these materials either for release as material free of radioactivity or as material for recovery in recycling processes.
Large-area proportional counters have heretofore been used preferentially for measuring surface contaminations. The possess a plane window made of as thin a metallized film as possible having a mass density of preferably about 1 mg/cm.sup.2, which extends over one face of a flat counter body or case for the entrance of radiation into the measuring volume. Readily available counter gases are used as measuring fluids. The operation of proportional counters is described e.g. by H. Neuert in "Kernphysikalische Meverfahren" , published by G. Braun, Karlsruhe, 1966.
Protection of the thin window is assured, as can be seen from Ref. Sci. Instr. Vol. 19, No. 11 (1948), pp. 733 -743, by an adjacent thin screen acting as a contact safety device, which may be drawn and attached like a cage around the longitudinal edges of large-area planar counters.
Such plane counters only have a limited field of view so that a tubular "film wall counter" with a bearing frame has already been proposed (DE-AS 1 071 242) in which a thin film hose is slipped over the front face of cylindrical insulators adequately spaced for stretching the film by means of a communicating support provided outside the film tube. Such film wall tubes are capable of covering larger solid angles. It has been found, however, that the film selected must be relatively thick to avoid problems arising in connection with elongations caused by the supply of counter gas or the effects of the electrical field between the metallized film and the counting wire. Very weak radiation is therefore hardly detected by such a probe.
The known proportional counters are thus not suited for adequately detecting contaminations accumulating preferentially in such locations as corners and angular spaces. A satisfactory approach to monitoring pipelines and channels has not been found either.